High intensity focused ultrasound transducer with acoustic lens

ABSTRACT

A high intensity focused ultrasound transducer includes an ultrasonic emitter having a surface that emits ultrasonic energy along a beam path, at least one low attenuation polymeric ultrasonic lens acoustically coupled to the surface in the beam path of the ultrasonic energy, such that the lens can direct the ultrasonic energy in at least one direction, and at least one stress mitigation feature, such as a kerf, a heat sink, or an acoustic matching layer, to mitigate thermal expansion mismatch stresses within the transducer. For manufacturing simplicity, the first surface is typically either flat or monotonically curvilinear. The lens may take a variety of shapes, including Fresnel features, and may focus, collimate, or defocus the ultrasonic energy. Any orientation and positioning of the at least one ultrasonic lens relative to the first ultrasonic emitter is contemplated. Manufacture is further simplified by molding, casting, or thermoforming the lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 11/646,526,filed 28 Dec. 2006, now abandoned, U.S. application Ser. No. 11/646,528,filed 28 Dec. 2006, U.S. application Ser. No. 11/642,923, filed 21 Dec.2006, U.S. application Ser. No. 11/642,821, filed 21 Dec. 2006, now U.S.Pat. No. 7,678,109, U.S. application Ser. No. 11/647,295, filed 29 Dec.2006, U.S. application Ser. No. 11/646,524, filed 28 Dec. 2006, now U.S.Pat. No. 7,699,845, and, and U.S. application Ser. No. 11/703,783, filed8 Feb. 2007. The foregoing applications are hereby incorporated byreference in their entirety as though fully set forth herein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The instant invention relates to ultrasound medical procedures. Inparticular, the instant invention relates to a lens-directed highintensity focused ultrasound (HIFU) transducer.

b. Background Art

High intensity focused ultrasound (HIFU) is a technique wherein thermaltherapy is typically delivered to a patient in the form of a focusedhigh power acoustic beam emanating from an acoustic transducer. Theprinciple advantage of focusing in HIFU is that the heating beam can befocused to selectively treat tissue regions, including remote interiortissues. Thus, HIFU is actively being developed for many treatments,such as cardiac ablation to treat cardiac arrhythmia and the destructionof cancerous tissues at depth.

One challenge in the design of reliable HIFU transducers is placing thetherapeutic heat at the desired location without allowing acousticheating or secondary, loss-related heating to damage the transducer ornon-targeted tissues or to interfere with the transducer's ongoingacoustic contact with tissue. Design of a HIFU transducer shouldtherefore take into consideration both the heat created by the primarytherapeutic acoustic energy directed upon or into the tissue and thewaste heat generated in the transducer due to imperfect (e.g., less than100%) electrical-to-acoustic energy conversion. One solution to thisproblem is to utilize a fluid-filled standoff membrane, which acts bothas a physical acoustic standoff and as a thermal sink for heat near thetransducer face. The fluid in the standoff may also be flowed orpermitted to weep in order to further cool the tissue.

There are three possible options for the construction of HIFUtransducers. First, a mechanically focused transducer, with a shapedpiezoemitter, may be employed. Second, a lens-focused transducer, with agenerally flat piezoemitter, may be employed. The term “lens” refers toan acoustically-redirecting entity through which acoustic energy passesand which provides a useful beam direction or reshaping, for example byfocusing the acoustic energy to one or more distal foci. Finally, anelectronically focused transducer, generally incorporating a generallyflat piezoemitter, may be utilized. All three options may furtherinclude an optional acoustic matching layer.

Virtually all extant acoustically focused therapeutic and surgicaltransducers are mechanically focused utilizing shaped piezoemitters.Such transducers are easy to design, of very high electroacousticefficiency because of the lack of acoustically lossy materials in thebeam path, and have negligible beam side-lobes because they typicallyhave only a single piezoelement. They are, however, quite expensive tomanufacture due to the complexity involved in shaping andsurface-finishing non-flat piezoelectric materials.

BRIEF SUMMARY OF THE INVENTION

Thus, it is desirable to be able to provide a relatively inexpensiveHIFU transducer that utilizes a generally flat acoustic emitteracoustically coupled to a molded acoustic lens.

It is also desirable to be able to achieve acoustic focusing of HIFUultrasound with minimal attenuation, and thus minimal thermal compromiseto the HIFU transducer.

It is further desirable to alleviate the thermal-expansion mismatchstresses that arise in the transducer during operation in order toprevent delamination or decoupling of the acoustic lens from theacoustic emitter, or other undesirable thermal damage effects.

It is also desirable to provide a HIFU transducer that is relativelysimple and economical to manufacture, and whose manufacture can bebatched and/or automated.

Lensed transducers are potentially more economical to manufacture due tothe relative simplicity of manufacturing a generally flat piezoemitterby comparison to a shaped piezoemitter, as well as the potential forcheaply molding a polymeric acoustic lens. However, because the acousticpower densities are quite high even at the transducer face and within anacoustic lens, it is clear that, if transducer focusing is to beprovided by an acoustic lens, the lens should have near zero acousticattenuation or it will melt, burn, or otherwise degrade. Further, due tothe differing thermal expansion coefficients between the lens materialand the piezoemitter material and/or matching layer material(s),thermal-expansion mismatch stresses may delaminate or otherwise decouplethe lens from the acoustic emitter and/or matching layer(s). Otherundesirable thermal effects are also prevalent, such as thermalexpansion mismatch-induced fracture or cracking of the piezocomponentsand lens eating, which can increase lens attenuation. The undesirablethermal effects are collectively referred to herein as “thermaldegradation” or “thermal compromise” of the HIFU transducer.

According to a first embodiment of the invention, a high intensityfocused ultrasound transducer includes: a first ultrasonic emitterhaving a first surface and a second surface opposite the first surface,the first ultrasonic emitter generating ultrasonic energy thatpropagates along a beam path from the first surface; at least onepolymeric ultrasonic lens acoustically coupled to the first surface atleast partially in the beam path of the ultrasonic energy propagatingtherefrom, such that the at least one polymeric ultrasonic lens candirect or shape the ultrasonic energy propagating from the firstultrasonic emitter in at least one direction; and at least one stressmitigation feature configured to mitigate thermal expansion mismatchstresses arising between the first ultrasonic emitter and the at leastone polymeric ultrasonic lens during operation of the transducer. Thetransducer may operate at a frequency between about 1.5 MHz and about 50MHz, and more preferably between about 2 MHz and about 30 MHz, while theultrasonic energy preferably has an instantaneous power density betweenabout 1000 W/cm² and about 5000 W/cm² at one or more points in the beampath. Such acoustic power densities at the acoustic focus are typicalfor thermal lesion-forming HIFU transducers.

At least one of, and optionally both of, the first surface willtypically be either generally flat or monotonically curvilinear.Optionally, the first ultrasonic emitter may be either plano-concave orplano-convex. Typically, the first ultrasonic emitter is a piezoelectricelement, such as a piezoceramic, PZT, a piezocomposite, or apiezopolymer, but it may also be an electrostrictive material, amagnetostrictive material, a ferroelectric material, an electrostaticelement, a micromechanical element, a micro-electro-mechanical element,or a combination thereof. As described in further detail below, anoptional stress mitigation layer, such as an acoustic matching layer, athermally conductive layer, or a stress-buffering layer, may overlay thefirst surface, or, in some embodiments, the second surface.

The at least one ultrasonic lens may focus, collimate, defocus, orotherwise shape, direct, or redirect the emitted ultrasonic energy in atleast one, and optionally in at least two, directions, and may, whenfocusing, direct the ultrasonic energy to a single focus or to aplurality of foci. For example, the lens may have a plano-concave crosssection with respect to a cross-sectional cut along one axis or alongtwo orthogonal axes. It may be either adjacent and directly coupled tothe first ultrasonic emitter or spaced apart from the first ultrasonicemitter with an ultrasonic transmission medium, such as an ultrasonictransmission fluid, disposed between the lens and the emitter. Typicallow-attenuation materials for the ultrasonic lens includepolyetherimides, polyetheretherketones, crosslinked polystyrenes,polyolefins, and any combinations thereof. The at least one ultrasoniclens may be a unitary piece, and may be either simple (i.e., a singlelens segment) or compound (i.e., multiple lens segments). One or moresegments of the lens may be Fresnel segments, plano-concave segments,plano-convex segments, or convex-concave segments. To minimize thermaldegradation and thermal compromise due to lens heating and/or expansion,the attenuation of the ultrasonic lens is preferably less than or equalto about 2 dB/cm-MHz, more preferably less than about 1.5 dB/cm-MHz, andmost preferably less than about 1 dB/cm-MHz when measured at about roomtemperature.

Any orientation of the at least one ultrasonic lens relative to thefirst ultrasonic emitter that shapes or directs the emitted ultrasonicenergy is contemplated. For example, the lens may be a Fresnel lenshaving a substantially flat surface and a structured surface oppositethe substantially flat surface, either surface of which may bejuxtaposed in face-to-face relationship with the first surface of thefirst ultrasonic emitter. Alternatively, the Fresnel lens may have astructured surface and a curvilinear surface opposite the structuredsurface, and either surface may be juxtaposed in face-to-facerelationship with the first surface of the first ultrasonic emitter.Preferably, the angle at which the ultrasonic energy passes into andthrough the at least one polymeric ultrasonic lens is less than acritical acoustic angle above which large losses to shear modes occur,and the maximum thickness of the at least one polymeric ultrasonic lensis small enough to avoid thermal compromise to the transducer due tolens attenuative self-heating.

The transducer optionally includes a second ultrasonic emittersubstantially identical to the first emitter, with the first and secondultrasonic emitters connected at an angle to deliver the ultrasonicenergy to the tissue from different directions. This arrangement,although structurally more complex than a single ultrasonic emitter,advantageously allows for the use of thinner lenses and providesincreased freedom as to allowable foci relative to the single emitterembodiment.

In some embodiments, the at least one polymeric ultrasonic lens includesa plurality of segments (or “lenslets”); a first segment of the at leastone polymeric ultrasonic lens may be acoustically coupled to the firstsurface of the first ultrasonic emitter, and a second segment of the atleast one polymeric ultrasonic lens may be acoustically coupled to thefirst surface of the second ultrasonic emitter. In other embodiments,the transducer includes at least two polymeric ultrasonic lenses; afirst polymeric ultrasonic lens may be acoustically coupled to the firstsurface of the first ultrasonic emitter and a second ultrasonic lens maybe acoustically coupled to the first surface of the second ultrasonicemitter.

An acoustic matching layer is optionally provided between the firstultrasonic emitter and the at least one polymeric ultrasonic lens tomitigate thermal expansion mismatch stresses. As will be described infurther detail below, the term “mitigate” encompasses, withoutlimitation, stress reduction, stress buffering, and stress avoidance.Preferably, the acoustic matching layer has an acoustic impedanceintermediate to that of the first ultrasonic emitter and the at leastone polymeric ultrasonic lens. The acoustic matching layer may beadjacent to and may directly acoustically couple the first ultrasonicemitter and the at least one polymeric ultrasonic lens.

The stress mitigation features, including, without limitation,stress-relieving kerfs, thermally conductive heat sink films or layers,and acoustically-passive stress buffer layers, may be provided anywherein, on, or interleaved with the first ultrasonic emitter, the at leastone polymeric ultrasonic lens, the acoustic matching layer, or acombination thereof. A thermally conductive film or layer may also beelectrically conductive, so as to additionally serve as an electrode forthe transducer.

Optionally, the transducer includes an acoustically transmissivemembrane, which may include a urethane-based polymeric material,containing an ultrasonic transmission fluid or medium. The membrane maybe disposed over the at least one polymeric ultrasonic lens or over thefirst ultrasonic emitter. The ultrasonic transmission fluid may beflowed in order to cool the transducer and/or tissue. The membrane andtransmission fluid themselves may also serve to direct the ultrasonicenergy. The ultrasonic transmission fluid may also flow through at leastone channel within or defined by the at least one polymeric ultrasoniclens or another component of the transducer to provide emitter cooling,lens cooling, and/or transducer cooling.

An acoustic reflector material may be disposed adjacent the secondsurface of the first ultrasonic emitter to inhibit ultrasonic energyemissions from the second surface. The acoustic reflector typicallyincludes a plurality of cavities or pores that are not transmissive ofultrasound, and may further include a gas layer or an air-like materiallayer disposed adjacent the second surface of the first ultrasonicemitter. Such materials have acoustic properties close to that of air,and thus inhibit “backwards” acoustic propagation.

An optional housing may enclose at least a portion of the firstultrasonic emitter. The at least one polymeric ultrasonic lens may beintegrated into the housing. Such a housing may provide a sealed, dryenvironment for the transducer and its electrodes, as well as protectionof patient and user from operational voltages.

According to another aspect of the invention, a method of ablatingtissue includes the steps of: exciting at least one ultrasonic emitterto generate high intensity ultrasonic energy along a beam path, the highintensity ultrasonic energy having an instantaneous power density of atleast about 1000 W/cm² at one or more locations within the beam path;directing the high intensity ultrasonic energy in at least one directionwith at least one polymeric ultrasonic lens positioned in the beam pathand acoustically coupled to the at least one ultrasonic emitter, the atleast one polymeric ultrasonic lens having an attenuation less thanabout 2 dB/cm-MHz when measured at about room temperature; anddelivering the directed high intensity ultrasonic energy to tissue to beablated while using at least one stress mitigation feature to mitigatethermal expansion mismatch stresses arising between the at least oneultrasonic emitter and the at least one polymeric ultrasonic lens. Thestep of exciting at least one ultrasonic emitter may include excitingthe at least one ultrasonic emitter to emit any of the following: highintensity ultrasonic energy at a frequency between about 2 MHz and about7 MHz and a power of about 80 W to about 150 W, preferably at afrequency of about 3.5 MHz and a power of about 130 W; high intensityultrasonic energy at a frequency between about 2 MHz and about 14 MHzand a power of about 20 W to about 80 W, preferably at a frequency ofabout 6 MHz and a power of about 60 W; and high intensity ultrasonicenergy at a frequency between about 3 MHz and about 16 MHz and a powerof about 2 W to about 20 W, preferably at a frequency of about 6 MHz anda power of about 15 W. During ablation, the temperature of one or moreof the at least one ultrasonic emitter and the at least one ultrasoniclens may be directly or indirectly monitored and/or regulated to remainbelow a thermal damage point (that is, a point at which the transducerbegins to experience undesirable thermal consequences). A temperaturesensor, such as a thermocouple or thermistor, may be employed to monitorthe temperature of the lens or other transducer component.

In still another embodiment of the present invention, a high intensityfocused ultrasound tissue ablation device includes a plurality oftransducers, and at least some of the plurality of transducers include:at least one ultrasonic emitter having a substantially flat firstsurface, wherein the at least one ultrasonic emitter generatesultrasonic energy that propagates along a beam path from the firstsurface, the ultrasonic energy having a power density of at least about1000 W/cm² at one or more locations within the beam path; and at leastone ultrasonic lens acoustically coupled to the first surface in thebeam path of the ultrasonic energy propagating therefrom, such that theat least one ultrasonic lens can direct the ultrasonic energy in atleast one direction in order to deliver it to the tissue to be ablated,and wherein an attenuation of the at least one ultrasonic lens is lowenough to prevent thermal damage to the at least one ultrasonic lensduring operation of the tissue ablation device. Thus, the transducersmay include one or more kerfs, one or more heat sinks or thermallyconductive features, one or more stress-buffering layers, one or moreacoustic matching layers, or a combination thereof in order to mitigatethermal expansion mismatch stresses.

Optionally, the at least one ultrasonic lens spreads the high intensityultrasonic energy in at least one direction such that energy emitted byadjacent ones of the plurality of transducers overlaps. This facilitatescreation of a substantially continuous lesion, for example about atleast a portion of a pulmonary vein.

Also disclosed herein is a high intensity focused ultrasound tissueablation device including a plurality of transducers, at least some ofwhich include: at least one ultrasonic emitter having a monotonicallycurvilinear first surface, wherein the at least one ultrasonic emittergenerates ultrasonic energy that propagates along a beam path from thesurface, the ultrasonic energy having a power density of at least about1000 W/cm² at one or more locations within the beam path; and at leastone ultrasonic lens acoustically coupled to the first surface in thebeam path of the ultrasonic energy propagating therefrom, such that theat least one ultrasonic lens can direct the ultrasonic energypropagating from the first ultrasonic emitter in at least one directionin order to deliver it to tissue to be ablated, and wherein anattenuation of the at least one ultrasonic lens is low enough to preventthermal damage to the at least one ultrasonic lens during operation ofthe tissue ablation device.

In still another embodiment of the invention, a high intensity focusedultrasound transducer includes: an ultrasonic emitter having a flatsurface that emits high intensity ultrasonic energy along a beam path,the high intensity ultrasonic energy having a power density of at leastabout 1000 W/cm² at one or more locations within the beam path; and aliquid lens acoustically coupled to the ultrasonic emitter. The liquidlens includes a membrane covering the surface and an ultrasonictransmission fluid disposed between the membrane and the surface. Themembrane is shaped such that, when the membrane is filled with theultrasonic transmission fluid, the liquid lens focuses the ultrasonicenergy emitted by the ultrasonic emitter. The ultrasonic transmissionfluid may also be a gel, such that a “gel lens” is coupled to theultrasonic emitter.

According to yet another aspect of the invention, a method ofmanufacturing a high intensity focused ultrasound transducer includesthe steps of: providing at least one ultrasonic emitter having a surfacecapable of emitting high intensity ultrasonic energy along a beam path,the high intensity ultrasonic energy having a power density of at leastabout 1000 W/cm² at one or more locations within the beam path;providing at least one low attenuation polymeric ultrasonic lensconfigured to direct ultrasonic energy passing therethrough, the atleast one low attenuation polymeric ultrasonic lens having anattenuation less than about 2 dB/cm-MHz when measured at about roomtemperature; and acoustically coupling the at least one polymericultrasonic lens to the at least one ultrasonic emitter such that the atleast one polymeric ultrasonic lens can direct the high intensityultrasonic energy emitted by the at least one ultrasonic emitter in atleast one direction without thermally degrading. The surface may besubstantially flat or monotonically curvilinear; if monotonicallycurvilinear, the surface may be manufactured as substantially flat andthen bent into a monotonically curvilinear configuration. The lens maybe molded, cast, thermoformed, or formed using other conventionalmethods, and the lens and emitter may be laminated together. At leastone stress mitigation feature may be introduced into the transducer toavoid or otherwise mitigate thermal expansion mismatch stresses.

An advantage of the present invention is that it utilizes lower costacoustic components having one or more surfaces that are at leastmanufactured as flat.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-section illustrating some general features of aHIFU transducer according to the present invention.

FIG. 2 illustrates additional general features of a HIFU transduceraccording to the present invention.

FIG. 3 is a cross-section of a HIFU transducer utilizing a Fresnelacoustic lens.

FIG. 4 illustrates a HIFU transducer incorporating a stress mitigationlayer, kerfs, and thermally conductive films as thermal expansionmismatch stress mitigation features.

FIG. 5 illustrates a HIFU transducer with an enclosed ultrasonicemitter.

FIGS. 6 a and 6 b are, respectively, end and side views of a HIFUtransducer according to one embodiment of the invention utilizing aplano-concave lens.

FIGS. 7 a and 7 b are, respectively, end and side views of a HIFUtransducer according to a second embodiment of the invention utilizing aFresnel plano-concave lens.

FIGS. 8 a and 8 b are, respectively, end and side views of a HIFUtransducer according to another embodiment of the invention utilizing arigid or semi-rigid shell-like lens, which includes a cavity containingan acoustically transmissive material.

FIGS. 9 a and 9 b are, respectively, end and side views of a HIFUtransducer according to a fourth embodiment of the invention utilizing arigid or semi-rigid shell-like lens with Fresnel features, whichincludes a cavity containing an acoustically transmissive material.

FIGS. 10 a and 10 b are, respectively, end and side views of a HIFUtransducer according to still another embodiment of the invention thatincludes multiple ultrasonic emitters and acoustic lenses.

FIGS. 11 a and 11 b are, respectively, end and side views of a HIFUtransducer according to yet a further embodiment of the invention thatdirects the ultrasonic energy in at least two directions.

FIG. 12 is an end view of yet another embodiment of the presentinvention illustrating the use of an intermediate stress-buffering orstress-shielding layer between the ultrasonic emitter and the ultrasoniclens.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a high intensity focused ultrasound(HIFU)-capable transducer incorporating one or more acoustic lenses. Theinvention will be described first with reference to the general featuresof such a transducer, and then several embodiments will be describedwith greater particularity. Though the invention will be described inconnection with HIFU applications, it is contemplated that the inventionmay also be practiced in non-HIFU applications, for example ultrasonicimaging applications.

FIG. 1 depicts an elevational cross-section of an exemplary HIFUtransducer 10. HIFU transducer 10 generally includes a first ultrasonicemitter 12 that generates ultrasonic energy, at least one polymericultrasonic lens 14, and at least one stress mitigation feature that isconfigured to mitigate thermal expansion mismatch stresses arisingbetween first ultrasonic emitter 12 and ultrasonic lens 14 duringoperation of HIFU transducer 10. HIFU transducer 10 typically operatesat a frequency between about 1.5 MHz and about 50 MHz, and preferablybetween about 2 MHz and about 30 MHz in a pulsed or continuous wavemanner.

The term “mitigate,” as used herein, encompasses stress buffering,stress reduction, and stress avoidance. The term “stress buffering”refers to shielding or masking a stress such that it does not pass intoa fragile component, for example as by inserting an intervening, durablestress-absorbing layer. “Stress reduction” refers to reducing stress atits source, for example as by providing stress relief features (e.g.,kerfs or slots), reducing temperatures in a transducer having differingthermal expansion rates (for example, as by providing a thermallyconductive layer), reducing thermal gradients, or choosing materialsthat have more closely matched thermal expansion behaviors.

First ultrasonic emitter 12 generally includes a first surface 16 and asecond surface 18, which is opposite first surface 16. For purposes ofthis disclosure, the labels “first surface 16” and “second surface 18”are used to refer to the surfaces of first ultrasonic emitter 12 facingtowards and away from a patient tissue surface 20, respectively; firstsurface 16 may also be thought of as the output face of first ultrasonicemitter 12, while second surface 18 may be thought of as the backside offirst ultrasonic emitter 12. Thus, the ultrasonic energy generated byfirst ultrasonic emitter 12 propagates towards tissue surface 20 along abeam path emanating from first surface 16. Preferably, the ultrasonicenergy has an instantaneous power density between about 1000 W/cm² andabout 5000 W/cm² at one or more points along the beam path (one of skillin the art will appreciate that the time-averaged power may be lower),for example at one or more foci (as used herein, the terms “focus” and“foci” encompass both discrete focal points and larger focal regions,including, for example, focus along a line in space).

Typically, first ultrasonic emitter 12 is a piezoelectric element, suchas a piezoceramic, lead-zirconate-titanate (PZT) monolithicpolycrystalline or single-crystal slab, a piezopolymer material such asPVDF, or a polycrystalline or single-crystal piezocomposite materialsuch as a diced PZT with polymeric-filled kerfs. It is alsocontemplated, however, that first ultrasonic emitter 12 may instead bean electrostrictive material, a magnetostrictive material, aferroelectric material, a photoacoustic material, one or moreelectrostatic elements, one or more micromechanical elements, or one ormore micro-electro-mechanical (MEM) elements. Combinations of any of theabove are also contemplated.

First ultrasonic emitter 12 may take any of a number of shapes andconfigurations within the scope of the present invention. Preferably,first surface 16 is either substantially flat or monotonicallycurvilinear. In embodiments including a monotonically curvilinear firstsurface 16, first surface 16 may initially be formed substantially flatand then, during manufacture or assembly of HIFU transducer 10, may bedeformed into a monotonically curvilinear configuration for use. Formingfirst surface 16 as substantially flat reduces the cost and complexityassociated with the manufacture of first ultrasonic emitter 12, whileintroducing a monotonic curve facilitates, to an extent, mechanicaldirection of the ultrasonic energy generated by first ultrasonic emitter12.

In the preferred embodiment of the invention, both first surface 16 andsecond surface 18 are substantially flat, thereby greatly reducing thecost and complexity associated with the manufacture of first ultrasonicemitter 12. In other embodiments, first ultrasonic emitter 12 isplano-concave (e.g., first surface 16 is substantially flat and secondsurface 18 is concave), while in still other embodiments, firstultrasonic emitter 12 is plano-convex (e.g., first surface 16 issubstantially flat and second surface 18 is convex). As with amonotonically curvilinear first surface 16, use of a plano-concave orplano-convex first ultrasonic emitter 12 facilitates mechanicaldirection of the ultrasonic energy without significant adverse effectson the cost or complexity associated with the manufacture of firstultrasonic emitter 12. One of ordinary skill in the art will understandhow to select an appropriate shape and material for first ultrasonicemitter 12 given a particular application of HIFU transducer 10.

Ultrasonic lens 14 is acoustically coupled to first surface 16 at leastpartially in the beam path (i.e., between first surface 16 and tissuesurface 20) such that ultrasonic lens 14 can direct or redirect theultrasonic energy propagating from first ultrasonic emitter 12 in atleast one direction, for example to one or more foci, and, in someembodiments of the invention, in at least two directions, which may ormay not overlap. Typical directions in which ultrasonic lens 14 directsthe ultrasonic energy are the elevational (shown in FIG. 1) and/orazimuthal directions, though it is contemplated that the energy may bedirected in one or more other directions as well. The terms “direct” and“redirect” include, but are not limited to, focusing the ultrasonicenergy, collimating the ultrasonic energy, and spreading, homogenizing,or defocusing the ultrasonic energy.

Ultrasonic lens 14 may include a single lens segment (as shown inFIG. 1) or a plurality of lens segments. The term “lens segment” (or“lenslet”) is used herein to refer to a portion of ultrasonic lens 14capable of directing or redirecting at least some of the ultrasonicenergy generated by first ultrasonic emitter 12 in at least onedirection. Thus, ultrasonic lens 14 may be simple or compound. For acompound ultrasonic lens 14, it is contemplated that each lens segmentmay be formed as a unitary piece, and the plurality of lens segmentsthereafter arranged in acoustic communication with first ultrasonicemitter 12 to form ultrasonic lens 14. Preferably, however, ultrasoniclens 14 is formed in its entirety as a single piece, for example bymolding, casting, or thermoforming, regardless of whether ultrasoniclens 14 is simple or compound, as a unitary ultrasonic lens 14manufacturing process minimizes the cost and complexity associated withthe manufacture of HIFU transducer 10, and may also permit simultaneousbulk manufacture of several ultrasonic lenses 14. It is alsocontemplated that ultrasonic lens 14 may be molded with a flat surfaceand bent or radiused during the assembly of transducer 10 such that theoriginal flat surface becomes somewhat curved. It is also contemplatedthat ultrasonic lens 14 may be formed directly upon the acousticcomponents (e.g., ultrasonic emitter 12 and any matching layers), forexample as by direct casting or molding.

Where distal focusing of the ultrasonic energy is desired, a compoundultrasonic lens 14 may be utilized to direct the ultrasonic energy toone or more foci. A compound ultrasonic lens 14 may also direct theenergy to a single focus, and the energy may be directed to arrive inphase or out of phase—that is, the ultrasonic energy can arrive at asingle point or along a single line on or within tissue 20 at differenttimes and/or from different directions, depending on the particularapplication of HIFU transducer 10. A compound ultrasonic lens 14 mayalso be arranged wherein different subsets of lenslets or lens segmentsfocus to different depths in the tissue along one or more spatial linesor surfaces.

An acoustically transmissive membrane 34, preferably made wholly orpartly of a urethane-based thin flexible polymeric material a few milsor less thick, may be provided situated over ultrasonic lens 14, with anultrasonic transmission medium 22 disposed between membrane 34 andultrasonic lens 14. As generally known in the art, ultrasonictransmission medium 22 and membrane 34 acoustically couple HIFUtransducer 10 to tissue 20, for example by providing a conformal wettedacoustic and thermal contact to tissue 20, and may also provide astandoff between HIFU transducer 10 and tissue 20. In addition,ultrasonic transmission medium 22 may be flowed to cool HIFU transducer10 and/or tissue surface 20. Typically, transmission medium 22 will besaline or water. Membrane 34 may further include weep holes (not shown)through which ultrasonic transmission medium 22 may purposely leak, forexample to allow wetting and/or cooling fluid to flow upon the surfaceof tissue 20. As an alternative to an enclosing membrane, ultrasonictransmission medium 22 may be laterally retained, but not necessarilycompletely enclosed, within an edge-defined water dam (not shown), whichadvantageously avoids any attenuation due to membrane 34. It is alsocontemplated that transmission medium 22 or another coolant may flowthrough passages within or defined by ultrasonic lens 14 or anothercomponent of transducer 10 (e.g., through flow passages in emitter 12).Such cooling, of course, reduces the temperatures within transducer 10,thereby mitigating thermal mismatch stresses and staving off thermalcompromise and thermal degradation.

As one of ordinary skill will recognize, the manner in which ultrasoniclens 14 directs or redirects the ultrasonic energy passing therethroughdepends upon not only the shape and orientation of ultrasonic lens 14,but also upon the acoustic velocity of the material or materials fromwhich ultrasonic lens 14 is made. Accordingly, it is contemplated thatsegments of ultrasonic lens 14 may be plano-convex, plano-concave,convex-concave (e.g., shell-shaped, such as shown in FIG. 2), meniscus,convex-convex, concave-concave, or any combinations thereof, either faceof which may be facing tissue 20 or emitter 12 without departing fromthe spirit and scope of the present invention. Further, in addition tofixed-radius lenses, parabolic, hyperbolic, and cylindrical lenses arealso contemplated.

Ultrasonic lens 14 may have one or more discrete focal points or one ormore spatially extended focal regions. Further, these different foci maybe achieved not only by shaping and selection of the lens material, butalso by frequency-changing methods wherein the focus location is afunction of driving frequency; either or both of ultrasonic lens 14 oremitter 12 may have such a dependence.

As will be discussed in further detail below, ultrasonic lens 14 may bemechanically coupled (e.g., bonded) to emitter 12, or may beacoustically coupled to emitter 12 by ultrasonic transmission medium 22,an approach which facilitates physically changeable lenses for a givenemitter.

Suitable materials for ultrasonic lens 14 include, but are not limitedto, polyetherimides, polyetheretherketones, crosslinked polystyrenes,polyolefins, and any combinations thereof, all of which have quite lowattenuation loss. Ultem®, a General Electric polymer, is well-suited foruse in ultrasonic lens 14, insofar as it has an attenuation that is lowat room temperature and that does not increase with temperature.Preferred materials include, without limitation, Ultem 1000 and Ultem1000EF.

One of ordinary skill will understand how to select an appropriate shapeand material for ultrasonic lens 14 and orient the lens relative tofirst ultrasonic emitter 12 given a particular application of HIFUtransducer 10 and a desired beam shape. For example, a low velocityplano-convex lens or a high velocity plano-concave lens may be used tofocus the ultrasonic energy, while a low velocity plano-concave lens ora high velocity plano-convex lens may be used to spread the ultrasonicenergy, though the invention is not limited to these configurations.

Ultrasonic lens 14 may be positioned adjacent and directly acousticallycoupled (i.e., mechanically coupled) to first ultrasonic emitter 12, asshown in FIG. 1. The embodiment of FIG. 1 shows that ultrasonic lens 14may be mechanically coupled to first ultrasonic emitter 12 alongsubstantially its entire length and width. Alternatively, as shown inFIG. 2, at least a portion of ultrasonic lens 14 may be spaced apartfrom and indirectly acoustically coupled to first ultrasonic emitter 12,with a suitable ultrasonic transmission medium 22, such as water,saline, or gel, disposed between first ultrasonic emitter 12 and thespaced-apart portion of ultrasonic lens 14. Thus, in the embodiment ofFIG. 2, ultrasonic lens 14 is directly mechanically and acousticallycoupled to first ultrasonic emitter 12 over only a relatively smalldistance and is mechanically decoupled along the rest of its length;optionally, HIFU transducer 10 may be constructed such that ultrasoniclens 14 is not mechanically coupled to first ultrasonic emitter 12 atall. Certain advantages of the spaced apart configuration illustrated inFIG. 2 will be explained in further detail below.

Preferably, ultrasonic lens 14 has low attenuation at both roomtemperature and elevated temperatures (e.g., operating temperatures),such that attenuative self-heating does not thermally damage orthermally degrade HIFU transducer 10, first ultrasonic emitter 12, orultrasonic lens 14, or cause unintended burning of nearby tissue 20.Undesirable thermal damage and degradation includes, but is not limitedto, significant softening or glass-transition of ultrasonic lens 14 in amanner causing acoustic or mechanical disruption, thermal mismatchbreakage of any component, thermal mismatch delamination of anycomponent, interfacial bond failure between components, permanentincreases in lossiness or attenuation due to delamination or bubbling ofan interlayer bonding material, thermal depoling of the piezomaterial,and other significant irreparable changes in the operating parameters ofHIFU transducer 10. Another advantage of a low attenuation ultrasoniclens 14 is that a greater percentage of the ultrasonic energy generatedby first ultrasonic emitter 12 will reach tissue 20 for treatment.Accordingly, ultrasonic lens 14 preferably has an attenuation, measuredat about room temperature and about 2 MHz, less than or equal to about 2dB/cm-MHZ, more preferably less than or equal to about 1.5 dB/cm-MHz,and most preferably less than or equal to about 1 dB/cm-MHz.

It is desirable to minimize the acoustic attenuation of ultrasonic lens14 such that, for a given lens design, a minimum of attenuative heatwill be generated in ultrasonic lens 14. By choosing a low attenuationmaterial, such as Ultem®, it is possible to minimize a maximum thicknessof the lens, thereby also minimizing the resulting maximum thermalmismatch stress between the lens and the emitter.

In terms of potential thermal degradation and thermal compromise, it isalso desirable to avoid excessive conversion of ultrasonic compressivewaves to shear waves and therefore heat—the acoustic analogue to totalinternal reflection and total attenuation in an optical lens. Thus, theangle at which the ultrasonic energy generated by first ultrasonicemitter 12 passes into and through ultrasonic lens 14 is preferably lessthan the known critical angle for the material of ultrasonic lens 14. Asone of ordinary skill in the art should appreciate, the critical anglemay be determined experimentally.

As ultrasonic lens 14 becomes thicker, its total path attenuationincreases. Thus, it is also desirable to minimize the average andmaximum thicknesses of ultrasonic lens 14, at least within the beam pathof the ultrasonic energy, to further reduce attenuation and heating. Forexample, to reduce the average and maximum thicknesses of ultrasoniclens 14 of FIG. 1 designed for a specific focal distance, the lens maybe segmented into Fresnel features or segments, as shown in FIG. 3. AFresnel lens 14 reduces the thickness of the lens relative to a moretraditional, non-Fresnel lens (that is, a plano-concave Fresnel lens isthinner than an ordinary plano-concave lens). The reduced thickness ofthe Fresnel lens configuration of FIG. 3 reduces the acousticattenuation of ultrasonic lens 14, and therefore the likelihood ofthermally damaging HIFU transducer 10 during operation, whilesimultaneously increasing the fraction of the ultrasonic energydelivered to tissue surface 20.

As can be understood by analogy to the optical arts, at least onesurface 24 of a Fresnel lens is a structured surface, which may beformed by building protrusions upon or cutting grooves into asubstantially flat lens surface. Advantageously, structured surface 24may also function as a thermal radiator, particularly if a fluid, suchas ultrasonic transmission fluid 22, is adjacent and/or flowed paststructured surface 24. Such flow also advantageously sweeps any bubblesout of structured surface 24. The opposite surface of the Fresnel lensmay be substantially flat, curvilinear, or also structured. FIG. 3 showsthe opposite surface as flat and bonded to emitter 12. Depending uponthe application of HIFU transducer 10, either surface may be juxtaposedin face-to-face relation with first surface 16 of first ultrasonicemitter 12. If desired, a suitable deformable or flowable ultrasonictransmission medium, such as urethane, water, or gel, may be disposedbetween first surface 16 and the facing surface of the Fresnel lens, forexample where it is desired to flow such coolant between emitter 12 andlens 14, or where it is desired to avoid rigidly mechanically bondinglens 14 to emitter 12.

Any number of Fresnel elements 26 (three of which are labeled in FIG. 3)may be incorporated into ultrasonic lens 14. As the number of Fresnelelements 26 increases, the overall thickness of ultrasonic lens 14 willgenerally decrease, which also decreases the average and maximumattenuation of ultrasonic lens 14. In addition, as the number of Fresnelelements 26 increases, the complexity of structured surface 24 maydecrease, since the shape of structured surface 24 may be approximatedwith straight-edged, rather than curved, elements 26. An upper limit onthe number of Fresnel segments may be dictated by the desirability ofhaving such segments be smaller than the wavelength of ultrasound in thelens material. For example, about 5 segments may be used for the lowerfrequencies of about 3 to about 7 MHz, while between about 7 to about 10Fresnel segments may be used at higher frequencies.

Heat transfer capacity away from transducer 10 or tissue 20 may befurther enhanced by flowing a fluid over, around the edges of, orthrough HIFU transducer 10. In some embodiments of the invention, thefluid flows through at least one lens-segment channel within or definedby ultrasonic lens 14. Dedicated non-focusing lens channels for coolantflow may also be provided. Ultrasonic lens 14 may also include one ormore pores, permeations, or permeability through which fluid may pass orwick, for example to deliver cooling and/or acoustic coupling to thetissue/lens interface. It should be understood that cooling of the lenssurface or tissue/lens interface also causes at-depth cooling in tissue20 by outward thermal conduction from tissue 20 towards transducer 10.Such tissue and interface cooling can be beneficial if one desires, at aparticular point in an ablation procedure, to assure that all thermaldamage is subsurface in nature. It is within the scope of the inventionto control the flow of fluid to beneficially manipulate the temperatureof either or both of transducer 10 and tissue 20.

Ultrasonic lens 14 will have a non-zero, positive total integratedattenuation, and thus HIFU transducer 10 will heat during operation, forexample via backwards thermal conduction from lens 14, therebypotentially generating mechanical stresses as first ultrasonic emitter12 thermally expands and/or contracts to a different extent or at adifferent rate than ultrasonic lens 14. Typically, a polymericultrasonic lens 14 will desire to expand upon heating more than emitter12, thereby putting emitter 12 in tension and potentially causingcracking, fracture, or warpage. In addition, the interfacial bondbetween emitter 12 and lens 14 at the interface is stressed, and couldalso delaminate or fail, as opposed to either or both of emitter 12 andlens 14 breaking. There are two primary sources of such stresses: (i)differing thermal expansion coefficients between first ultrasonicemitter 12 material(s) and ultrasonic lens 14 material(s); and (ii)thermal gradients entirely within one or more of ultrasonic emitter 12material(s) and ultrasonic lens 14 material(s). In steady stateoperation, source (i) is typically the problem, whereas source (ii) istypically the problem during pulsed operation of HIFU transducer 10. Useof more thermally conductive components and the use of coolant may helpmitigate thermal mismatch stresses. In addition, the use of ramped-uppower, rather than a delta-function, may also beneficially reducetransient peak stresses.

The present invention contemplates a number of design features,illustrated in FIG. 4, that may be implemented to mitigate suchstresses. As shown in FIG. 4, HIFU transducer 10 may include one or morestress mitigation layers 28 (one such layer 28 is shown in FIG. 4),which may include, without limitation, acoustic matching layers,antimatching layers, foundation layers, thermally conductive layers, oracoustically-passive stress-buffering layers. Of course, a single layer28 may fulfill more than one purpose (e.g., a layer that is both anacoustic matching layer and a stress-buffering layer, or a layer that isboth a thermally conductive layer and a foundation layer).

Layers 28 are preferably selected and configured to mitigate thermalexpansion mismatch stresses arising between first ultrasonic emitter 12and ultrasonic lens 14 during operation of HIFU transducer 10. Forexample, a rigid and strong acoustic matching layer or a rigid andstrong passive buffer layer (e.g., a material that is not easilystress-damaged, or strained, such as ceramic, glass, or low-expansionmetal) effectively mechanically shields first ultrasonic emitter 12 fromultrasonic lens 14, thereby buffering any thermal expansion mismatchstresses that arise and reducing the risk of cracking first ultrasonicemitter 12 or debonding first ultrasonic emitter 12 and ultrasonic lens14. Preferably, a buffer layer (whether or not the buffer layer is alsoan acoustic matching layer) has a coefficient of thermal expansion thatis between the thermal expansion coefficients of ultrasonic lens 14 andemitter 12. More preferably, the coefficient of thermal expansion of abuffer layer is about equal to the coefficient of thermal expansion offirst ultrasonic emitter 12. A buffer layer typically has a sufficientlyhigh fracture toughness to withstand the stresses that arise in HIFUtransducer 10 during operation.

Layer 28 is preferably thermally conductive in order to convey thermalenergy away from either or both of first ultrasonic emitter 12 andultrasonic lens 14, thereby reducing thermal expansion mismatch stressesby reducing heating, and thus the amount of thermal expansion stresscoupled between first ultrasonic emitter 12 and ultrasonic lens 14. Morepreferably, layer 28 is more thermally conductive than first ultrasonicemitter 12 and ultrasonic lens 14.

A preferred material for layer 28 is aluminum nitride, which is a highlythermally conductive, low expansion material that can serve as anacoustic matching layer, a thermally conductive layer, and also as astress-buffering layer. The following table provides the thermalproperties of some suitable layers 28 as compared to the materials foremitter 12 and ultrasonic lens 14:

Material α in/in/C K W/m-K Vitr Carbon 2.5~3.5 × 10⁻⁶    4.6~6.3 Alumina6.7 × 10⁻⁶ 37 Glass 10.48 × 10⁻⁶  1.38 AlN 4.6 × 10⁻⁶ 175 SiC 3.7 × 10⁻⁶272 Macor 9.3 × 10⁻⁶ 1.46 PZT (emitter) 3.8~4.5 × 10⁻⁶    1.1 Ultem ®1000 (lens)  54 × 10⁻⁶ 0.22

As illustrated in FIG. 4, layer 28 acoustically couples ultrasonic lens14 to first ultrasonic emitter 12. Layer 28 may be directly acousticallyand mechanically coupled via bonding or joining, analogous to FIG. 1, orindirectly via a transmissive spacer of standoff material, such asultrasonic transmission medium 22, analogous to FIG. 2. That is, layer28 may be adjacent to or spaced apart from either or both of ultrasoniclens 14 and first ultrasonic emitter 12. Where layer 28 is an acousticmatching layer, it will typically have an acoustic impedanceintermediate to the acoustic impedances of first ultrasonic emitter 12and ultrasonic lens 14.

Those familiar with acoustic design will realize that one couldalternatively implement backside (e.g., second surface 18) antimatchingor reflective layers (not shown) or metallic mass-load layers, which areplaced to remove heat or to provide a rigid foundation for emitter 12.Most favorably, any appreciably thick metal layer, whether frontside orbackside, will be thermally conductive and of modest expansioncoefficient. Invar™ and Kovar™ nickel-iron based alloys are suitable forthis purpose. Of course, in addition to providing a stable flat orshaped surface to emitter 12, a foundation layer may also provide one ormore of a heat removal path, an acoustic matching layer, an antimatchinglayer, an attenuative backer, or an electrode.

Typical materials for an acoustic matching layer include, but are notlimited to, aluminum nitride, boron nitride, silicon nitride, graphite,vitreous carbon, silicon carbide, cermets, glasses, some metals, andsome polymers, as well as mixtures or composites thereof. Thermallyand/or electrically conductive microparticles or nanoparticles may alsobe used, particularly as dispersed or mixed into a composite materialmatching layer that is polymer, glass, ceramic, or metal-matrix based.Lens 14 and layer 28 might also be combined as a premade laminate of twodifferent materials or compositions, yielding a configuration lookingsimilar to that illustrated in FIGS. 1 and 3 after assembly. One ofordinary skill in the art will understand how to select and configureone or more suitable layers 28, such as acoustic matching layers,antimatching layers, foundation layers, thermally conductive layers, andstress-buffering layers for a particular application of HIFU transducer10.

The accumulated stress in a thermally mismatched interface of twomaterials, such as the direct-bonded interface between first ultrasonicemitter 12 and ultrasonic lens 14 in FIG. 3, is proportional to theaccumulated distance over which that mismatch exists, in addition tobeing generally proportional to the temperature and the intrinsicexpansion mismatch per unit temperature itself. Thus, one or more stressrelief kerfs 30 (FIG. 4) may be provided in first ultrasonic emitter 12and/or ultrasonic lens 14. For purposes of this disclosure, a “kerf” isany disruption of an otherwise contiguous span of material, and reducesstress by reducing the accumulated distance over which thermal mismatchexists within HIFU transducer 10. Any number of kerfs 30, in anydirection, may be employed. By appropriately locating kerfs 30 withinHIFU transducer 10, thermal mismatch stresses may be mitigated (e.g.,relieved) without significantly compromising acoustic and thermalperformance. Preferably, layers 28 also serve as a stress-mitigating(e.g., stress buffering) backbone for HIFU transducer 10, and thus doesnot include kerfs, but it is within the scope of the invention toinclude kerfs in layers 28 in addition to or instead of kerfs in eitheror both of first ultrasonic emitter 12 and ultrasonic lens 14. It shouldbe understood that the term “kerf,” as used herein, is not limited to adicing cut in a transducer, but rather refers to any disruption inmaterial, whether created additively (e.g., by juxtaposing lens oremitter segments with intermediate gaps) or subtractively (e.g., bycutting into the lens or the emitter).

FIG. 4 illustrates kerfs 30 located generally centrally to transducer 10along the azimuthal direction. Frequently, one will choose to bisect ortrisect the overall length of transducer 10.

Thermally-induced expansion mismatch stresses may be further mitigatedthrough the use of a heat sink feature, such as one or more thermallyconductive layers 28 or films 32, shown in FIG. 4, disposed to conductheat outwardly from an interior region of HIFU transducer 10. Thermallyconductive films 32 or layers 28 reduce the magnitude of thermalgradients within HIFU transducer 10 and/or reduce the peak temperatureof HIFU transducer 10. Films 32 may be electroplated, evaporated, orsputtered, or may be interleaved metal foils such as those used inflexible circuits. Preferably, however, thermally conductive films 32are deposited using physical or chemical vapor deposition techniques.Films 32 are preferably more conductive thermally than emitter 12 andultrasonic lens 14.

Thermally conductive films 32 may be provided on numerous interior orexterior surfaces of HIFU transducer 10 or any of its layers/components,including, but not limited to, first and second surfaces 16, 18 of firstultrasonic emitter 12 and either face of ultrasonic lens 14. In additionto being thermally conductive, films 32 may also be electricallyconductive, such that they may also serve as electrodes for firstultrasonic emitter 12, electrically insulative, or partiallyelectrically conductive and partially electrically insulative. One ofordinary skill in the art will also appreciate that suitably configuredfilms 32 may replace one or more layers 28.

In some embodiments of HIFU transducer 10, either or both of membrane 34and ultrasonic transmission medium 22 disposed therein may contribute todirecting or redirecting the ultrasonic energy generated by firstultrasonic emitter 12. That is, either or both of membrane 34 andultrasonic transmission medium 22 disposed therein may serve as a“liquid lens” or “gel lens” (if ultrasonic transmission medium 22 is agel) that focuses, collimates, or defocuses the ultrasonic energy.Suitable fluids for such a “liquid lens” include, but are not limitedto, fluoropolymeric liquids and perfluorocarbon liquids. Preferably,such a fluid would likely be circulated or captured in a closedenclosure rather than permitted to flow into the patient.

Referring now to FIG. 5, a housing 36 is shown enclosing at least aportion of first ultrasonic emitter 12. As shown, ultrasonic lens 14 isintegrated with housing 36, though it is also contemplated that housing36 may be formed separately from ultrasonic lens 14. Housing 36 sealsfirst ultrasonic emitter 12 from water and other fluids and includes acover 38 sealed to housing 36. Although the exterior surfaces of housing36 are shown as generally flat and parallel to each other, they may haveany shape without departing from the spirit and scope of the presentinvention. It is a manufacturing advantage to be able to mold the lensand the housing as one entity. It is also within the spirit and scope ofthe invention to mold lens 14 in any manner that includes otherfunctional features of a transducer 10, for example a compound lens 14that is molded to house multiple emitters 12.

An acoustic reflector 40 may be provided adjacent second surface 18 offirst ultrasonic emitter 12. Acoustic reflector 30 inhibits ultrasonicenergy emissions from second surface 18 (that is, propagating away fromtissue 20), thereby increasing the operational efficiency of HIFUtransducer 10. In general, acoustic reflector 40 includes one or morematerials that are not transmissive of ultrasound, such as gas- orair-filled gaps, pores, or cavities, unwettable pseudo-air foams, andvacuum, any of which may be sealed against second surface 18 withinhousing 36 if desired. In addition to inhibiting “backwards” acousticpropagation, an unwettable pseudo-air foam advantageously prevents fluidingress into transducer 10 without requiring transducer 10 to behermetically sealed, which in turn reduces the cost of manufacturingtransducer 10.

Certain specific embodiments of a HIFU transducer will now be describedwith reference to FIGS. 6-12. One of ordinary skill in the art willappreciate that additional combinations of the various elements,features, and orientations disclosed herein are possible, and willunderstand how to select and orient the various elements and featuresdescribed in designing a transducer for a particular application. Thus,the embodiments of the present invention may include any number orcombination of the foregoing design aspects.

FIG. 6 a illustrates an end (azimuthal) view of a HIFU transducer 60,while FIG. 6 b illustrates HIFU transducer 60 in side (elevational)view. HIFU transducer 60 includes a single substantially flat ultrasonicemitter 62 and a single plano-concave ultrasonic lens 64 directlyacoustically and mechanically coupled thereto. Acoustic matching layer28 couples ultrasonic emitter 62 to ultrasonic lens 64. To mitigatethermal expansion mismatch stresses arising in HIFU transducer 60, bothultrasonic emitter 62 and ultrasonic lens 64 include kerfs 30 in boththe elevational and azimuthal directions. Kerfs 30 cut ultrasonicemitter 62 and ultrasonic lens 64 substantially in half both azimuthallyand elevationally, but do not substantially penetrate acoustic matchinglayer 28, such that acoustic matching layer holds HIFU transducer 60together. Kerfs 30, if narrow (e.g., a few mils wide), do not have asignificant effect on the ability of ultrasonic lens 64 to directultrasonic energy, as they are small relative to the overall size ofultrasonic lens 64.

In addition, a plurality of thermally conductive films 32, shown in FIG.6 a, are provided on several surfaces within HIFU transducer 60,including both faces of ultrasonic emitter 62 and both faces ofultrasonic lens 64, in order to conduct heat outwardly from HIFUtransducer 60. As shown with dashed lines in FIGS. 6 a and 6 b, theultrasonic energy generated by ultrasonic emitter 62 is focused byultrasonic lens 64 in the elevational plane (FIG. 6 a) to create a lineof focus in the azimuthal plane (FIG. 6 b), and this is referred to as“cylindrically focused” along the azimuthal direction.

FIGS. 7 a and 7 b illustrate a HIFU transducer 60′ according to apreferred embodiment of the invention that is functionally analogous toHIFU transducer 60. However, plano-concave ultrasonic lens 64 has beenreplaced by a plano-concave Fresnel ultrasonic lens 64′ that includesthree Fresnel elements 26 arranged in the elevational plane. To minimizethe effect of kerfs 30 on direction of the ultrasonic energy, kerfs 30are preferably placed at the junction between Fresnel elements 26. Theaverage thickness of Fresnel ultrasonic lens 64′ is less than theaverage thickness of ultrasonic lens 64, thereby lowering total acousticattenuation, generating less total lens-attenuative heat, and reducingthermal expansion mismatch stresses arising between ultrasonic emitter62 and Fresnel ultrasonic lens 64′ of FIGS. 7 a and 7 b relative tothose arising between ultrasonic emitter 62 and ultrasonic lens 64 ofFIGS. 6 a and 6 b. Assuming that the Fresnel segments retain the surfacecurvature of the original lens 64 would focus in the same manner as lens64. Of course, as one of ordinary skill in the art will appreciate, anylens can be configured to direct or redirect the ultrasonic energy asdesired.

FIGS. 8 a and 8 b illustrate a HIFU transducer 70 where the ultrasonicenergy generated by ultrasonic emitter 72 is directed by a shell-likeconvex-concave ultrasonic lens 74. Typically, lens 74 would have aconvex radius different from its concave radius and thus variablethickness to provide focusing action. As shown in dashed lines in FIGS.8 a and 8 b, the ultrasonic energy is focused along the azimuthaldirection to create a line of focus in the azimuthal plane. As bestshown in FIG. 8 a, a portion of convex-concave ultrasonic lens 74 isindirectly acoustically coupled to ultrasonic emitter 72 via ultrasonictransmission medium 22 disposed between ultrasonic lens 74 and acousticmatching layer 28; ultrasonic lens 74 is only mechanically coupled anddirectly acoustically coupled to acoustic matching layer 28 overrelatively short lengths 76 a, 76 b, which are preferably, but notnecessarily, peripheral to the beam path. By substantially mechanicallydecoupling ultrasonic lens 74 from ultrasonic emitter 72, the effect ofaccumulated interfacial thermal expansion mismatch stresses iseffectively limited to lengths 76 a, 76 b. Further, since lengths 76 a,76 b are preferably peripheral to the beam path, the mechanical couplingbetween ultrasonic lens 74 and ultrasonic emitter 72 may be arranged tobe elastic or lossy in order to further mitigate thermal expansionmismatch stresses. Further mitigation of harmful thermal effects may beprovided by flowing ultrasonic transmission medium 22.

FIGS. 9 a and 9 b illustrate a HIFU transducer 70′ according to anotherpreferred embodiment of the invention that is functionally analogous toHIFU transducer 70 as described in connection with FIGS. 8 a and 8 b.However, convex-concave ultrasonic lens 74 has been replaced by afocus-equivalent convex-concave Fresnel ultrasonic lens 74′ thatincludes three Fresnel elements 26. Thus, HIFU transducer 70′ is to HIFUtransducer 70 as HIFU transducer 60′ is to HIFU transducer 60.

FIGS. 10 a and 10 b illustrate a HIFU transducer 80 that incorporatestwo ultrasonic emitters 82 a, 82 b, each with a correspondingplano-concave ultrasonic lens 84 a, 84 b acoustically coupled thereto.Ultrasonic emitters 82 a, 82 b are angled relative to each other.Preferably, this angle is between about 5 degrees and about 45 degrees,more preferably between about 20 degrees and about 35 degrees. As shownin dashed lines, emitters 82 a, 82 b are each cylindrically focused inthe elevation plane along the azimuthal direction. Ultrasonic emitters82 a, 82 b can be activated to deliver ultrasonic energy to the focus orfoci either in phase or out of phase. Typically, the two focal lines ofemitters 82 a, 82 b will be arranged to overlap in space. One ofordinary skill in the art will appreciate that HIFU transducer 80 couldalso be modified such that ultrasonic lenses 84 a, 84 b are replaced byequivalent Fresnel lenses. Further, though FIGS. 10 a and 10 billustrate independent ultrasonic lenses 84 a, 84 b coupled toultrasonic emitters 82 a, 82 b, it is contemplated that one could moldthe two lenses 84 a, 84 b as a single, contiguous entity (not shown).

An intrinsic acoustic advantage of the device illustrated in FIGS. 10 aand 10 b is that each lens 84 a, 84 b needs to individually redirect thebeam in the elevation plane to a lesser amount than would a singleemitter/lens transducer of equal total elevation width focused at thesame depth. Thus, lenses 84 a, 84 b can be thinner and run cooler, andtherefore be less thermally stressed. Acoustic practitioners will alsorecognize that the device of FIGS. 10 a and 10 b provides twoindependent means of forming a focus—tilt angle and lens design—leadingto improved flexibility.

FIGS. 11 a and 11 b illustrate, respectively, elevational and azimuthalviews of a HIFU transducer 90 generally similar to HIFU transducer 60′as described above in connection with FIGS. 7 a and 7 b, but configuredto also angularly direct the ultrasonic energy generated by ultrasonicemitter 92 in the azimuthal plane. This, transducer 90 exhibits beamdirection in both the elevational plane and the azimuthal plane. Due tothe curvature of ultrasonic lens 94 along the azimuthal direction in theazimuthal plane, the beam is deflected outwards by an angle Θ, as wellas propagating forward. Such a configuration is particularly desirable,for example, in a tissue ablation device including a plurality of HIFUtransducers 90 placed end-to-end along the azimuthal direction. Onetypically uses such a stringed device to create a substantiallycontinuous lesion, for example an ablation lesion intended to isolateall or part of one or more pulmonary veins, such as disclosed in U.S.Pat. No. 6,805,128 to Pless et al. One of ordinary skill in the art willbe familiar with the construction and function of such a tissue ablationdevice. By spreading the ultrasonic energy in the azimuthal direction,any potential gaps between HIFU transducers 90 can be targeted withultrasonic energy (that is, adjacent ones of the plurality of HIFUtransducer 90 ablation elements will have overlapping beams), therebyfurther facilitating creation of a substantially continuous lesionwithout the need to move the ablation device. The particular curvatureof ultrasonic lens 94 may be adjusted such that the tissue in which thelesion is formed receives a generally uniform amount of ultrasonicenergy.

FIG. 12 illustrates a HIFU transducer 100 incorporating an ultrasonicemitter 102 and an ultrasonic lens 104 integrated into housing 36. Anintermediate compliance layer 106 is disposed between ultrasonic emitter102 and ultrasonic lens 104. Compliance layer 106 is a stress mitigationfeature, and is typically a material that flows or easily deforms inresponse to stress, such as a gel or Indium metal. Such a compliancelayer can also serve as an acoustic matching layer and/or a thermalsinking layer. Note that the purpose of a compliance layer is to allowsome local stress-relieving strain (compliance) to take place, whereasthe stress-buffering layer described above prevents strain from takingplace. In either case, however, the purpose of beneficially mitigatingthermal mismatch stresses is served.

To manufacture a HIFU transducer according to the present invention, atleast one ultrasonic emitter having a surface capable of emitting highintensity ultrasonic energy along a beam path and at least one lowattenuation polymeric ultrasonic lens configured to direct or redirectultrasonic energy passing therethrough are provided. The at least oneultrasonic emitter and the at least one polymeric ultrasonic lens arethen acoustically coupled, for example by laminating or otherwisebonding the lenses to the emitters, such that the at least one polymericultrasonic lens can direct or redirect the high intensity ultrasonicenergy emitted by the at least one ultrasonic emitter in at least onedirection without succumbing to thermal degradation or thermalcompromise. One or more stress mitigation features or thermal conductionfeatures, such as the stress-buffering layers, matching layers,thermal-sinking layers, compliance layers, kerfs, and heat sinksdescribed at length above, may also be introduced. Of course, any layermay be arranged so as to also act as an acoustic matching layer orelectrode.

As described above, the ultrasonic emitter is preferably substantiallyflat as manufactured, preferably on both major faces, but at least onone major face. This permits ultrasonic emitters to bebatch-manufactured as a slab that can thereafter be cut to form betweenabout 10 and about 20 individual emitters at a time. Any acousticmatching layer, which is also preferably substantially flat, can besimilarly batch-processed. A plurality of ultrasonic lenses maysimilarly be batch molded, cast, or thermoformed. By utilizing batchprocessing, one is not required to handle large numbers of relativelysmall parts until just before or at transducer lamination, providing asubstantial manufacturing advantage. Furthermore, if the transducers arelaminated at low temperatures or using ultraviolet-curing adhesives,several connected transducers may be batch processed for simultaneouslamination and divided thereafter. That is, a slab of ultrasonicemitters may be bonded to a slab of acoustic matching layer and a slabof ultrasonic lenses, and then the slab may be separated to form aplurality of HIFU transducers, which may then be assembled into amedical device such as a tissue ablation device.

It is also contemplated that, in addition to the lens, molding orcasting manufacturing technologies may be applied to acoustic matchinglayers, stress-buffering layers, thermal-sinking layers, or compliancelayers.

HIFU energy delivered by one or more HIFU transducers according to thepresent invention may be used to ablate tissue, for example in thetreatment of cardiac arrhythmia. Thus, at least one ultrasonic emittermay be excited to generate high intensity ultrasonic energy along a beampath. The energy so generated may be directed (e.g., focused,collimated, or defocused) in at least one direction via at least one lowattenuation polymeric ultrasonic lens positioned in the beam path andacoustically coupled to the ultrasonic emitter. The directed highintensity ultrasonic energy is then delivered to the tissue to beablated, either to a single focus or to a plurality of foci, which maybe on, beneath, or behind the tissue surface adjacent the HIFUtransducer. During the ablation procedure, the operating temperature ofone or more of the ultrasonic emitters and the ultrasonic lenses may bedirectly or indirectly monitored and regulated to remain below a thermaldamage point, for example by flowing an ultrasonic transmission mediumthrough the transducer to provide cooling thereto. The HIFUtransducer(s) may be designed to have one or more localized or extendedfocal regions at one or more transducer operating conditions. Thetransducers may even be arranged to deliver thermally-conductive heatingand lesioning to surface tissues at locations of weak acoustic focus.

An ablation device incorporating HIFU transducers according to thepresent invention, such as a belt- or wand-type ablation device for useon cardiac tissue, preferably delivers ultrasonic energy focused in atleast one plane or to at least one point. Most commonly, in order toform an extended lesion, the transducers will focus the acoustic energyalong an azimuthal direction. In particular, the ablation devicepreferably delivers focused ultrasound having a focal depth of about 2mm to about 20 mm, more preferably of about 2 mm to about 12 mm, andmost preferably of about 8 mm. Stated another way, a focus is spacedapart from the interface of the HIFU transducer and the tissue beingtreated along a focal axis (FA) within the stated ranges. The focusedultrasound also forms an angle of about 10 degrees to about 170 degrees,more preferably of about 30 degrees to about 90 degrees, and mostpreferably of about 60 degrees relative to the FA. Each HIFU transducerpreferably has a length of about 0.43 inch, a width of about 0.35 inch,and a thickness of about 0.017 inch.

It should be understood that a transducer may be arranged to focus atone depth or at multiple depths over a focal range. Further, by varyingfrequency, the user can vary how much energy attenuates before reachingthe focus. For example, a higher frequency will attenuate faster overdepth than a lower frequency. This effect can be leveraged in amulti-step ablation algorithm, such as described below. Further, focican also be mechanically moved by moving the transducer relative to thetissue, for example as by changing the inflated dimension of asaline-filled standoff or membrane. In addition, a multi-segment lenscan be arranged, if desired, to have subsets of its lens segmentsfocused at different depths, operating at the same or differentfrequencies.

An advantage of using focused ultrasonic energy for tissue ablation isthat the energy can be concentrated within the tissue at depth. Anotheradvantage of using focused ultrasound is that the directed energydiverges and reduces intensity after traveling beyond the focus, therebyreducing the possibility of damaging tissue beyond the target tissuedepth as compared to more collimated ultrasonic energy. When ablatingepicardial tissue with collimated ultrasound, the collimated ultrasoundenergy, if not strongly attenuated, is not absorbed by the immediatelyadjacent target tissue and travels through the heart chamber and remainsconcentrated on a relatively small area when it reaches the endocardialsurface on the other side of the chamber. The present invention reducesthe likelihood of damage to other structures since the ultrasonic energydiverges beyond the focus and is spread over a larger area at anydownstream impact point. As touched on above in the discussion of FIGS.11 a and 11 b, the ultrasonic energy may be produced by a number of HIFUtransducers oriented to focus or concentrate ultrasonic energy, such asat least about 90% of the energy, within preferred angle ranges andradii of curvature. In another aspect of the invention, the transducersmay be operated during two different time periods while varying at leastone characteristic, such as the frequency of the ablating energy, thepower of the ablating energy, the position of the focus relative to thetissue, and/or the ablating time. For example, HIFU transducers may beoperated at varying frequencies over time to ablate tissue in acontrolled manner. Specifically, the HIFU transducers are preferablyoperated to create a transmural lesion by controlling the delivery ofenergy to the tissue. Although it is preferred to vary the frequencywhen ablating the tissue, the HIFU transducers may, of course, beoperated at a single frequency without departing from the spirit andscope of the invention.

In a first treatment method of the present invention, the transducer isactivated at a frequency of about 2 MHz to about 7 MHz, and preferablyof about 3.5 MHz, and a power of about 80 watts to about 150 watts, andpreferably of about 130 watts, in short bursts. For example, thetransducer may be activated for about 0.01 second to about 2.0 seconds,and preferably for about 1.2 seconds. The transducer is inactive forabout 2 seconds to about 90 seconds, more preferably about 5 seconds toabout 80 seconds, and most preferably about 45 seconds betweenactivations. In this manner, a controlled amount of accumulated energycan be delivered to the tissue in short bursts to heat tissue at andnear the focus while minimizing the impact of blood cooling at theendocardium. Ablation at this frequency may continue until a controlledamount of energy is delivered, such as about 0.5 kilojoule to about 3kilojoules. Treatment at this frequency in relatively short burstsproduces localized heating at the focus. At the first frequency, energyis not absorbed as quickly in the tissue as it is at higher frequencies,so that heating at the focus is not significantly affected by absorptionof ultrasound energy in tissue before reaching the focus.

Typically, in order to lesion the endocardium against and despite thecooling blood of the blood pool, one will deliver an adiabatic ornear-adiabatic heating pulse as close to the tissue/blood interface aspossible. Preferably, the heating pulse will be delivered slightlyinside the tissue adjacent the blood pool. “Adiabatic” means that theacoustic attenuation heating is delivered faster than it has a chance toappreciably conduct away from its focal target. Typical adiabaticdelivery involves short pulses, frequently on the order of a second, afraction of a second, or even measured in milliseconds, which times areshorter than a thermal relaxation time of the target tissue. One mayalso beneficially precede this pulse with a non-adiabatic preheating toincrease the target tissue several degrees, such that the adiabaticpulse has less overall heating to do. Typically, the acoustic powerdensity at the focus will be between about 1000 W/cm² and about 5000W/cm².

Following treatment at the first frequency, the transducer is operatedfor longer periods of time, preferably about 1 second to about 4seconds, and more preferably about 2 seconds, to ablate tissueintermediately between the focus and the transducer. The frequencyduring this treatment is also preferably about 2 MHz to about 14 MHz,more preferably about 3 MHz to about 7 MHz, and most preferably about 6MHz. The transducer is operated for about 0.7 second to about 4 secondsat a power of about 20 watts to about 80 watts, and preferably about 60watts. The transducer is inactive for between about 3 seconds and about60 seconds, and preferably for about 40 seconds, between eachactivation. In this manner, a controlled amount of energy can bedelivered to heat tissue intermediately between the focus and thetransducer. The treatment at this frequency may continue until acontrolled amount of total energy is delivered, such as about 750joules.

As a final treatment stage, the ultrasonic transducer is activated at ahigher frequency to heat and ablate the near surface. The transducer ispreferably operated at a frequency of between about 3 MHz and about 16MHz, and preferably at about 6 MHz. The transducer is operated at lowerpower than the treatment methods above since the ultrasonic energy israpidly absorbed by the tissue at these frequencies, so that the nearsurface is heated quickly. In a preferred method, the transducer isoperated at about 2 watts to about 20 watts, and more preferably about15 watts. The transducer is preferably operated for a sufficientduration to ablate tissue, such as about 20 seconds to about 80 seconds,and preferably about 40 seconds. Often, the near surface temperaturewill reach about 70 degrees C. to about 85 degrees C.

Each of the treatments described above may be used by itself or incombination with other treatments. Furthermore, the combination oftransducer size, power, frequency, activation time, and focal length mayall be varied to produce the desired delivery of ultrasound energy tothe tissue. As such, it is understood that the preferred embodiment maybe adjusted by adjusting one or more of the characteristics and, thus,these parameters may be changed without departing from the spirit andscope of the invention. The treatment sequence described above generallydelivers energy closer to the near surface during the second treatmentand even closer to the near surface for the third treatment (that is, itablates tissue from the far surface towards the near surface insuccessive treatments).

The focus of the ultrasound energy may also be moved relative to thetissue to deliver energy to different depths in the tissue. The HIFUtransducer can be moved closer to and farther away from the targettissue, for example via variable membrane water-inflation, for example,with membrane 34 conforming to the required shape to fill the gapbetween the transducer and the tissue. Membrane 34 is preferablyinflated, for example utilizing a pressurized fluid such as saline, anddeflated to mechanically move the focus in this manner. However, thetransducer may also be moved with any other suitable mechanism, such aslifting it from the tissue via a threaded foot, preferably locatedoutside the beam. The focus may be moved or scanned while thetransducers are activated or may be moved between activations of thetransducers. Moving the focus of the ultrasound energy may be sufficientto create a transmural lesion without changing frequencies, or may beused in conjunction with a change in frequencies as described above. Thefocus may also be moved in any other manner such as with a phased arrayor variable acoustic lensing, such as a palette of interchangeableultrasonic lenses from which the physician can choose before, or evenduring, the procedure.

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the spirit or scope of this invention. For example, one skilled inthe art will appreciate that the labels used herein to describe thesurfaces of first ultrasonic emitter 12 are merely a matter ofconvenience and could be reversed or altered without departing from thespirit and scope of the present invention (that is, it is within thespirit and scope of the present invention for ultrasonic energy toemanate from second surface 18 instead of, or in addition to, firstsurface 16).

Further, though all transducers described herein were generallyrectangular in shape, the present invention is applicable to transducersof any shape, including rotationally symmetric transducers.

In addition, though the present invention has been described in thecontext of HIFU transducers utilized to provide ablation therapy to apatient, it is within the spirit and scope of the invention to apply theprinciples disclosed herein to other applications, such as metrology andimaging.

All directional references (e.g., upper, lower, upward, downward, left,right, leftward, rightward, top, bottom, above, below, vertical,horizontal, clockwise, and counterclockwise) are only used foridentification purposes to aid the reader's understanding of the presentinvention, and do not create limitations, particularly as to theposition, orientation, or use of the invention. Joinder references(e.g., attached, coupled, connected, and the like) are to be construedbroadly and may include intermediate members between a connection ofelements and relative movement between elements. As such, joinderreferences do not necessarily infer that two elements are directlyconnected and in fixed relation to each other.

It is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeonly and not limiting. Changes in detail or structure may be madewithout departing from the spirit of the invention as defined in theappended claims.

1. A high intensity focused ultrasound transducer, comprising: a firstultrasonic emitter having a first surface and a second surface oppositesaid first surface, said first ultrasonic emitter generating ultrasonicenergy that propagates along a beam path from said first surface; atleast one polymeric ultrasonic lens acoustically coupled to said firstsurface at least partially in the beam path of the ultrasonic energypropagating therefrom, such that said at least one polymeric ultrasoniclens can direct the ultrasonic energy propagating from the firstultrasonic emitter in at least one direction; and at least one stressmitigation feature configured to mitigate thermal expansion mismatchstresses arising between said first ultrasonic emitter and said at leastone polymeric ultrasonic lens during operation of said transducer. 2.The transducer according to claim 1, wherein at least one of the firstsurface and the second surface is substantially flat.
 3. The transduceraccording to claim 2, wherein said first ultrasonic emitter is aplano-concave ultrasonic emitter.
 4. The transducer according to claim2, wherein said first ultrasonic emitter is a plano-convex ultrasonicemitter.
 5. The transducer according to claim 2, wherein both the firstsurface and the second surface are substantially flat.
 6. The transduceraccording to claim 1, wherein at least one of the first surface and thesecond surface is monotonically curvilinear.
 7. The transducer accordingto claim 1, wherein said at least one polymeric ultrasonic lens isadjacent and directly coupled to said first ultrasonic emitter.
 8. Thetransducer according to claim 1, wherein at least a portion of said atleast one polymeric ultrasonic lens is spaced apart from said firstultrasonic emitter, and further comprising an ultrasonic transmissionmedium disposed between said at least one polymeric ultrasonic lensportion and said first ultrasonic emitter.
 9. The transducer accordingto claim 1, wherein said at least one polymeric ultrasonic lens isconfigured to focus the ultrasonic energy in at least one direction. 10.The transducer according to claim 1, wherein said at least one polymericultrasonic lens is configured to collimate the ultrasonic energy in atleast one direction.
 11. The transducer according to claim 1, whereinsaid at least one polymeric ultrasonic lens is configured to defocus theultrasonic energy in at least one direction.
 12. The transduceraccording to claim 1, wherein said at least one polymeric ultrasoniclens directs the ultrasonic energy propagating from the first ultrasonicemitter in at least two directions.
 13. The transducer according toclaim 1, wherein an acoustic transmission attenuation of said at leastone polymeric ultrasonic lens measured at about room temperature is lessthan or equal to about 2 dB/cm-MHz.
 14. The transducer according toclaim 13, wherein the acoustic transmission attenuation is less than orequal to about 1.5 dB/cm-MHz.
 15. The transducer according to claim 14,wherein the transmission attenuation is less than or equal to about 1.0dB/cm-MHz.
 16. The transducer according to claim 1, wherein said firstultrasonic emitter comprises a piezoelectric element.
 17. The transduceraccording to claim 1, wherein said first ultrasonic emitter is selectedfrom the group consisting of piezoceramic materials, piezopolymermaterials, piezocomposite materials, electrostrictive materials,magnetostrictive materials, ferroelectric materials, electrostaticelements, micromechanical elements, micro-electro-mechanical elements,and any combinations thereof.
 18. The transducer according to claim 1,further comprising a foundation layer mechanically coupled to saidsecond surface of said first ultrasonic emitter, the foundation layerproviding at least one of a stable flat surface, a stable shapedsurface, a heat removal path, an acoustic matching layer, an acousticantimatching layer, an attenuative backer, or an electrode.
 19. Thetransducer according to claim 1, further comprising a stress-bufferinglayer located between and mechanically coupled to said first ultrasonicemitter and said at least one polymeric ultrasonic lens, wherein saidstress-buffering layer is configured to mitigate thermal expansionmismatch stresses arising between said first ultrasonic emitter and saidat least one polymeric ultrasonic lens during operation of saidtransducer.
 20. The transducer according to claim 19, wherein thestress-buffering layer has a coefficient of thermal expansion aboutequal to a coefficient of thermal expansion of the first ultrasonicemitter.
 21. The transducer according to claim 1, further comprising anacoustic matching layer located between said first ultrasonic emitterand said at least one polymeric ultrasonic lens, wherein said acousticmatching layer is further configured to mitigate thermal expansionmismatch stresses arising between said first ultrasonic emitter and saidat least one polymeric ultrasonic lens during operation of saidtransducer, and wherein said acoustic matching layer acousticallycouples said first ultrasonic emitter and said at least one polymericultrasonic lens.
 22. The transducer according to claim 21, wherein athermal conductivity of said acoustic matching layer is greater than athermal conductivity of at least one of said first ultrasonic emitterand said at least one polymeric ultrasonic lens.
 23. The transduceraccording to claim 21, wherein said matching layer has an acousticimpedance intermediate to an acoustic impedance of said first ultrasonicemitter and an acoustic impedance of said at least one polymericultrasonic lens.
 24. The transducer according to claim 21, wherein saidacoustic matching layer is adjacent to and directly acoustically couplessaid first ultrasonic emitter and said at least one polymeric ultrasoniclens.
 25. The transducer according to claim 21, wherein said acousticmatching layer further comprises at least one stress mitigation featureconfigured to mitigate thermal expansion mismatch stresses arisingbetween said first ultrasonic emitter and said at least one polymericultrasonic lens during operation of said transducer.
 26. The transduceraccording to claim 1, further comprising a backside anti-matching layermechanically coupled to said second surface of said first ultrasonicemitter.
 27. The transducer according to claim 1, further comprising amass-load layer configured to remove heat from said transducer.
 28. Thetransducer according to claim 1, wherein said at least one polymericultrasonic lens is a unitary piece.
 29. The transducer according toclaim 1, wherein said at least one polymeric ultrasonic lens is made ofa material selected from the group consisting of polyetherimides,polyetheretherketones, crosslinked polystyrenes, polyolefins, and anycombinations thereof.
 30. The transducer according to claim 1, whereinsaid at least one polymeric ultrasonic lens comprises a plurality oflens segments.
 31. The transducer according to claim 30, wherein saidplurality of lens segments are configured to direct the ultrasonicenergy to a single focus.
 32. The transducer according to claim 30,wherein said plurality of lens segments are configured to direct theultrasonic energy to a plurality of foci.
 33. The transducer accordingto claim 30, wherein at least one segment of said at least one polymericultrasonic lens is a Fresnel lens segment.
 34. The transducer accordingto claim 33, wherein said Fresnel lens segment includes a substantiallyflat surface and a structured surface opposite said substantially flatsurface, and wherein said substantially flat surface is juxtaposed inface-to-face relation with said first surface of said first ultrasonicemitter.
 35. The transducer according to claim 33, wherein said Fresnellens segment includes a curvilinear surface and a structured surfaceopposite said curvilinear surface, and wherein said curvilinear surfaceis juxtaposed in face-to-face relation with said first surface of saidfirst ultrasonic emitter.
 36. The transducer according to claim 33,wherein said Fresnel lens segment includes at least one structuredsurface juxtaposed in face-to-face relation with said first surface ofsaid ultrasonic emitter, and further comprising an ultrasonictransmission medium disposed between said Fresnel lens segment and saidfirst surface of said ultrasonic emitter.
 37. The transducer accordingto claim 30, further comprising: a second ultrasonic emitter having afirst surface and a second surface opposite said first surface, saidsecond ultrasonic emitter generating ultrasonic energy that propagatesalong a beam path from said first surface, wherein a first segment ofsaid at least one polymeric ultrasonic lens is acoustically coupled tosaid first surface of said first ultrasonic emitter and in the beam pathof the ultrasonic energy propagating therefrom, such that said firstsegment can direct the ultrasonic energy propagated from said firstultrasonic emitter in at least one direction, and wherein a secondsegment of said at least one polymeric ultrasonic lens is acousticallycoupled to said first surface of said second ultrasonic emitter and inthe beam path of the ultrasonic energy propagating therefrom, such thatsaid second segment can direct the ultrasonic energy propagated fromsaid second ultrasonic emitter in at least one direction.
 38. Thetransducer according to claim 1, further comprising: a second ultrasonicemitter having a first surface and a second surface opposite said firstsurface, said second ultrasonic emitter generating ultrasonic energythat propagates along a beam path from said first surface; and at leasttwo polymeric ultrasonic lenses, wherein a first polymeric ultrasoniclens is acoustically coupled to said first surface of said firstultrasonic emitter and in the beam path of the ultrasonic energypropagating therefrom, such that said first polymeric ultrasonic lenscan direct the ultrasonic energy propagating from the first ultrasonicemitter in at least one direction; and wherein a second polymericultrasonic lens is acoustically coupled to said first surface of saidsecond ultrasonic emitter and in the beam path of the ultrasonic energypropagating therefrom, such that said second polymeric ultrasonic lenscan direct the ultrasonic energy propagating from the second ultrasonicemitter in at least one direction.
 39. The transducer according to claim1, wherein said at least one polymeric ultrasonic lens comprises atleast one plano-concave lens segment.
 40. The transducer according toclaim 1, wherein said at least one polymeric ultrasonic lens comprisesat least one plano-convex lens segment.
 41. The transducer according toclaim 1, wherein said at least one polymeric ultrasonic lens comprises aconvex-concave lens, wherein at least a portion of said convex-concavelens is spaced apart from said first surface of said first ultrasonicemitter, and further comprising an ultrasonic transmission mediumdisposed between said first surface of said ultrasonic emitter and saidconvex-concave lens portion.
 42. The transducer according to claim 1,wherein said at least one ultrasonic lens comprises an acousticallytransmissive membrane over said first ultrasonic emitter and anultrasonic transmission fluid disposed between said membrane and saidfirst ultrasonic emitter.
 43. The transducer according to claim 42,wherein at least one of said membrane and said ultrasonic transmissionfluid provides acoustic lensing.
 44. The transducer according to claim1, further comprising an acoustically transmissive membrane over said atleast one polymeric ultrasonic lens and a flowable ultrasonictransmission medium disposed between said at least one polymericultrasonic lens and said membrane, wherein the ultrasonic transmissionmedium provides one or more of transducer cooling, tissue cooling, andtransducer standoff from tissue.
 45. The transducer according to claim44, wherein said membrane comprises a urethane-based polymeric material.46. The transducer according to claim 1, wherein said at least onestress mitigation feature comprises at least one kerf in at least one ofsaid first ultrasonic emitter or said at least one polymeric ultrasoniclens.
 47. The transducer according to claim 1, wherein said at least onestress mitigation feature comprises at least one heat sink featureconfigured to reduce one or more of a thermal gradient and a peaktemperature in said transducer.
 48. The transducer according to claim47, wherein said heat sink feature comprises a thermally conductivematerial disposed to conduct heat outwardly from an interior region ofsaid transducer, wherein a thermal conductivity of said thermallyconductive material is greater than a thermal conductivity of at leastone of said first ultrasonic emitter and said at least one polymericultrasonic lens.
 49. The transducer according to claim 48, wherein saidthermally conductive material is also electrically conductive.
 50. Thetransducer according to claim 1, further comprising an acousticreflector material disposed adjacent said second surface of said firstultrasonic emitter, wherein said acoustic reflector material inhibitsultrasonic energy emissions from said second surface.
 51. The transduceraccording to claim 50, wherein said acoustic reflector comprises areflective layer mechanically coupled to said second surface of saidfirst ultrasonic emitter.
 52. The transducer according to claim 51,wherein said reflective layer further provides a rigid foundation forsaid first ultrasonic emitter.
 53. The transducer according to claim 50,wherein said acoustic reflector includes a plurality of cavities thatare not transmissive of ultrasound.
 54. The transducer according toclaim 1, further comprising a gas disposed adjacent said second surfaceof said first ultrasonic emitter, wherein said gas inhibits ultrasonicenergy emissions from said second surface.
 55. The transducer accordingto claim 1, further comprising a housing enclosing at least a portion ofsaid first ultrasonic emitter.
 56. The transducer according to claim 55,wherein said housing and said at least one polymeric ultrasonic lens areformed as a unitary component.
 57. The transducer according to claim 1,wherein said at least one polymeric ultrasonic lens defines at least onechannel through which a fluid may flow, wherein said fluid provides oneor more of emitter cooling, lens cooling, and transducer cooling. 58.The transducer according to claim 1, wherein the angle at which theultrasonic energy passes into and through said at least one polymericultrasonic lens is less than a critical angle.
 59. The transduceraccording to claim 1, wherein a maximum thickness of said at least onepolymeric ultrasonic lens is sufficiently small that attenuative heatingof said at least one polymeric ultrasonic lens by the ultrasonic energypassing therethrough is insufficient to thermally damage saidtransducer.
 60. The transducer according to claim 1, wherein saidtransducer operates at an operating frequency between about 1.5 MHz and50 MHz.
 61. The transducer according to claim 60, wherein saidtransducer operates at an operating frequency between about 2 MHz andabout 30 MHz.
 62. The transducer according to claim 1, wherein theultrasonic energy has a power density between about 1000 watts/cm² andabout 5000 watts/cm² at one or more points in the beam path.
 63. A highintensity focused ultrasound transducer, comprising: an ultrasonicemitter having a flat surface that emits high intensity ultrasonicenergy along a beam path, the high intensity ultrasonic energy having apower density of at least about 1000 W/cm² at one or more locationswithin the beam path; and a liquid lens acoustically coupled to saidultrasonic emitter, said liquid lens comprising: a membrane coveringsaid surface; and an ultrasonic transmission fluid disposed between saidmembrane and said surface, wherein said membrane is shaped such that,when said membrane is filled with said ultrasonic transmission fluid,said liquid lens focuses the ultrasonic energy emitted by saidultrasonic emitter.
 64. The transducer according to claim 63, whereinsaid ultrasonic transmission fluid is a fluoropolymeric liquid.
 65. Thetransducer according to claim 63, wherein said ultrasonic transmissionfluid is a perfluorocarbon liquid.
 66. A method of manufacturing a highintensity focused ultrasound transducer, comprising the steps of:providing at least one ultrasonic emitter having a surface capable ofemitting high intensity ultrasonic energy along a beam path, the highintensity ultrasonic energy having a power density of at least about1000 W/cm² at one or more locations within the beam path; providing atleast one low attenuation polymeric ultrasonic lens configured to directultrasonic energy passing therethrough, the at least one low attenuationpolymeric ultrasonic lens having an attenuation less than about 2dB/cm-MHz when measured at about room temperature; and acousticallycoupling the at least one polymeric ultrasonic lens to the at least oneultrasonic emitter, whereby the at least one polymeric ultrasonic lenscan direct the high intensity ultrasonic energy emitted by the at leastone ultrasonic emitter in at least one direction without thermallydegrading.
 67. The method according to claim 66, wherein the step ofproviding at least one ultrasonic emitter having a surface capable ofemitting high intensity ultrasonic energy along a beam path comprisesproviding at least one ultrasonic emitter having a substantially flatsurface capable of emitting high intensity ultrasonic energy along abeam path.
 68. The method according to claim 66, wherein the step ofproviding at least one ultrasonic emitter having a surface capable ofemitting high intensity ultrasonic energy along a beam path comprisesproviding at least one ultrasonic emitter having a monotonicallycurvilinear surface capable of emitting high intensity ultrasonic energyalong a beam path.
 69. The method according to claim 68, wherein thestep of providing at least one ultrasonic emitter having a monotonicallycurvilinear surface capable of emitting high intensity ultrasonic energyalong a beam path comprises bending a substantially flat ultrasonicemitter into a monotonically curvilinear configuration.
 70. The methodaccording to claim 66, wherein the step of providing at least one lowattenuation polymeric ultrasonic lens configured to direct ultrasonicenergy passing therethrough comprises forming the at least one polymericultrasonic lens as a unitary piece.
 71. The method according to claim66, wherein the step of acoustically coupling the at least one polymericultrasonic lens to the at least one ultrasonic emitter comprisesmechanically coupling the at least one polymeric ultrasonic lens to theat least one ultrasonic emitter.
 72. The method according to claim 71,further comprising introducing at least one stress mitigation featureconfigured to mitigate thermal-expansion mismatch stresses arisingbetween said at least one polymeric ultrasonic lens and said at leastone ultrasonic emitter during operation of said transducer.